The present invention relates to a method for producing coated workpieces according to the preamble of Claim 1, to uses therefor according to Claims 28 to 35, to an installation for implementing the above-mentioned method according to the preamble of Claim 36 and to uses therefor according to Claims 51 to 54.
The present invention is based on problems which occur during the manufacturing of thin layers by means of CVD and PECVD methods. The findings made in this case, according to the invention, can be applied particularly to the production of semiconductor layers, for example, when producing solar cells or modulation doped FETs or hetero-bipolar transistors.
Thin semiconductor films are deposited either in a monocrystalline form, that is, epitaxially, on an also monocrystalline substrate, such as a silicon substrate, or are deposited in a polycrystalline form or amorphous form on polycrystalline or amorphous substrates, such as glass. Although in the following the invention will be described mainly with respect to the production of silicon-coated and/or germanium-coated substrates, it may, as mentioned above, also be used for the production of other workpieces and workpieces coated with other materials.
Known methods for depositing epitaxial semiconductor films are:
Molecular beam epitaxy (MBE),
chemical vapor deposition (CVD),
remote plasma enhanced CVD with DC or HF discharge,
electron cyclotron resonance plasma-assisted CVD. (ECRCVD).
xe2x80x9cCVD methodxe2x80x9d is a collective term for a large number of thermal deposition methods which differ either in the construction of the assigned apparatuses or in their operating mode. Thus, for example, a CVD method can be carried out at a normal atmospheric pressure or at much lower pressures down into the range of the ultra high vacuum. Reference can be made in this respect to (1) as well as to (2).
In the commercial production of epitaxial Si layers, only CVD is normally used. In this case, the applied reactive gases are silicon-containing gases, such as silane chlorides, SiCl4, Si,HCl and SiH2Cl2 as well as silanes, such as SiH4, or Si2H4. Characteristics of the standard CVD methods are the high deposition temperatures in the order of 1,000xc2x0 and more, as well as pressures of typically 20 mbar to 1,000 mbar, that is, to normal atmospheric pressure.
Translator""s note: The subscripts on this page are only guesses since most are illegible in the German. 
According to the process conditions, coating rates of several xcexcm per minute can be achieved in this manner. corresponding to several 100 xc3x85/sec., with respect to which reference is again made to (1).
In contrast, low pressure chemical vapor deposition (LPCVD), which is synonymous with low pressure vapor phase epitaxy (LPVPE), takes place at pressures below 1 mbar and permits lower process temperatures to typically 700xc2x0 C. In this respect, reference is made, in addition to (1), also to (3) and (6).
With respect to the LPCVD and with reference to (6), at a deposition temperature of 650xc2x0 C., a growth rate of
GR=50 xc3x85/min
is indicated. This takes place at a reactive gas flow for silane of
F=14 sccm.
This results in a characteristic number which is relevant to the gas yield, specifically the growth rate per reactive gas flow unit GRF at
GRF=3.6 xc3x85/(sccmxc2x7min)
On 5xe2x80x3 wafers, corresponding to a surface
AS=123 cm2,
converted from the actual surface A2 for 2xe2x80x3 wafers, a deposition quantity (growth amount) GA is obtained at
GA=5.2xc2x71014 Si atoms/sec.
Again, with respect to a reactive gas flow unit, the characteristic number xe2x80x9cdeposition quantity per reactive gas flow unitxe2x80x9d, in the following called xe2x80x9cgas utilization numberxe2x80x9d, GAF is obtained at
GAF=8.4xc2x710xe2x88x923,
corresponding to 8.4 o/oo.
At 650xc2x0, an epitaxial layer is formed.
If the deposition temperature is reduced to 600xc2x0 C., a polycrystalline layer is formed. In this case, the following applies:
GR=3 xc3x85/min
F=28 sccm silane
GRF=0.11 xc3x85/sccm/min)
GA=3.1xc2x71015 is Si atoms/sec on AR 
GAF=2.5xc2x710xe2x88x924, corresponding to 0.25 o/oo.
Basically, the following criteria are required for a defect-free epitaxial layer growth:
In the case of transmission electron microscopy on cross-sectional preparations, the proof of epitaxy is established by electron diffraction and high resolution.
In the area of 10 to 15 xcexcm, which in this case can typically be penetrated by radiation, along the boundary surface to the substrate, no defects must be visible. Typical enlargements in the analysis of defects are 110,000 to 220,000.
Another development is the ultra high vacuum chemical vapor deposition (UHV-CVD) with working pressures in the range of 10xe2x88x924 to 10xe2x88x922 mbar, typically in the range of 10xe2x88x923 mbar, with respect to which reference is made to (4) as well as to (5), (7). It permits very low workpiece temperatures; however, the growth rates or coating rates being extremely low; thus, for example, approximately 3 xc3x85/min for pure silicon at 550xc2x0 C. according to (5).
The reason for the low growth rates is the fact that the absorption rate and decomposition rate of the reactive molecules, thus, for example, of SiH4, decreases with an increasing hydrogen coating of the workpiece surface. The layer growth is therefore limited by the desorption rate of H2, which, however, rises exponentially with the temperature. In this respect, reference is made to (8). Because of the lower bonding energy of the Gexe2x80x94H bonding in comparison to the Sixe2x80x94H bonding, the hydrogen desorption of an Sixe2x80x94Ge alloy surface is higher, so that, while the substrate temperature is the same, a higher growth rate is obtained than in the case of pure Si; for example, at a content of 10% Ge by a factor 25 at 550xc2x0 C. (5).
Another possibility of achieving high deposition rates of an epitaxy quality at low substrate temperatures consists of (9) decomposing the reactive gases by means of a u-wave plasma (ECRCVD).
By the use of plasma sources, which are based on the principle of electron cyclotron resonance, the incidence of high-energy ions onto the substrate is to be avoided.
As a rule, such sources operate in the pressure range of 10xe2x88x923 to 10xe2x88x924 mbar, which, however, results in larger free path lengths than in the case of capacitively coupled-in high-frequency Hf plasmas. This, in turn, can lead to an undesirable ion bombardment of the substrate and thus to the generating of defects, as indicated in (10). The energy of the ions impacting on the substrate, however, can be limited by an external control of the substrate potential, whereby ion-related damage can largely be avoided. Also by means of the ECRCVD method, the growth rates for pure silicon, as a rule, amount only to a few 10 xc3x85/min, at low deposition temperaturesxe2x89xa6600xc2x0 C.
Summarizing, this results in the following:
Layers which are deposited with a quality which is suitable also for the depositing of epitaxial layers can be deposited at deposition temperaturesxe2x89xa6 up to now:
by UHV-CVD with growth rates GR of approximately 3 xc3x85/min or
ECRCVD with a growth rate GR higher by approximately 1 order (30 xc3x85/min).
PECVD methods, whose plasmas are produced by DC discharges, could be used for the manufacturing of layers of epitaxy qualityxe2x80x94that is, a correspondingly lower fault density (see above)xe2x80x94neither for the construction of epitaxial nor for the construction of amorphous or polycrystalline layers; at least not with a growth rate GR, reliability and efficiency to be ensured for industrial manufacturing.
On the other hand, the use of capacitively coupled-in high-frequency fields for generating HF plasmas for PECVD methods was reported very early, with respect to which reference is made to (11). The difficulty of this approach is the fact that not only the reactive gases are decomposed in such Hf plasmas. Simultaneously, the substrate surface is exposed to an intensive bombardment of highly energetic ions, as utilized specifically also in the case of reactive atomizing or high-frequency etching. This, on the one hand, promotes the hydrogen desorption but, simultaneously results in defects in the growing layers. A method, which is modified in this respect, the RPCVDxe2x80x94remote plasma chemical vapor depositionxe2x80x94takes this into account in that the substrates to be coated are not exposed directly to the HF plasma, which leads to better results (12). However, the achieved growth rates are low, specifically usually fractions of nm per minute to no more than several nm per minute according to (13).
It is an object of the present invention to indicate a method which can be used in industrial manufacturing and which permits the growing of layers of an epitaxy quality which have significantly higher growth rates than previously known.
This is achieved by means of methods of the initially mentioned type which are characterized according to the characterizing part of Claim 1 and by a system which is characterized according to the characterizing part of Claim 36. Preferred embodiments of the method are specified in Claims 2 to 27; preferred embodiments of the system are specified in Claims 37 to 50. The method according to the invention is particularly suitable for the manufacturing of semiconductor-coated substrates with an epitaxial, amorphous or polycrystalline layer, in this case particularly of Si, Ge or Si/Ge alloy layers as well as Ga or Ga bonding layers.
In this case, particularly also doped semiconductor layers can be deposited; layers containing silicon and/or germanium, doped preferably with at least one element of Groups III or V of the classification of elements or layers containing gallium with at least one element of Groups II, III, IV or VI of the classification of elements, for example, with Mg or Si.
Concerning the initially discussed coating techniques for producing epitaxial layers, the following can be summarized:
The CVD methods, particularly the UHV-CVD methods, lead to excellent layer qualities even at substrate temperatures below 500xc2x0 C. They are therefore suitable for also producing epitaxial layers, where extremely high demands are made on the layer quality. However, in the case of this method, the growth rate, for example, for Si, is extremely low, as mentioned above, in the order of 3 xc3x85/min at 550xc2x0 C.
Microwave-plasma-assisted methods, ECRCVD, have the advantage that the decomposition of the reactive molecules can take place without high thermal energy. The ion bombardment of the substrate leads to an increased hydrogen desorption. Both effects can result in a considerable increase of the growth rate. However, at low temperatures, unacceptably high defect densities are observed which are induced by the ion bombardment. Although a control by way of the substrate bias voltage increases the layer quality, it does not change the comparatively low rates.
Thus, there seems to be an inherent contradiction: An ion bombardment of the substrate, on the one hand, leads to an increased growth rate because of an increased hydrogen absorption, but simultaneously increases the defect density.
The following picture exists according to (2) for thermal CVD methods operated at atmospheric pressure:
Si growth rate GR: 2xc3x9710xe2x88x923 nm/min (at 600xc2x0 C., measured 3xc2x710xe2x88x922 and converted to 550xc2x0 C.)
Gas flow, SiCl2H2, F: 100 sccm.
This results in a growth rate GR per SiCl2H2 flow unit, GRF2xc3x9710xe2x88x924 xc3x85/(sccmxc2x7min).
A gas flow F of 100 sccm SiCl2H2 corresponds to 4.4xc3x971019 molecules/sec.
The growth rate GR of 2xc3x9710xe2x88x923 nm/min corresponds to a growth rate of 2xc3x9710xe2x88x924 silicon monolayers per second on an Sxe2x80x3 wafer, corresponding to a surface A5 of 123 cm2. Thus, on the total surface, a deposited quantity of
GA=1.7xc3x971013 silicon atoms/sec.
is obtained per second. By relating the silicon quantity deposited per second and the reactive gas quantity admitted per second, the gas utilization number GAF is obtained at
xe2x80x83GAF=3.9xc3x9710xe2x88x927.
This corresponds to a utilization of approximately 0.0004 o/oo.
We note that, at atmospheric CVD, the following is obtained:
GRF≈2xc3x9710xe2x88x924 xc3x85/(sccmxc2x7min)
GAF≈0.0004 o/oo.
From (5), combined with (4) and (7), the following estimate is obtained for UHV-CVD:
GRF≈0.1 xc3x85/(sccmxc2x7min) and
GAF≈0.0035 corresponding to approximately 35 o/oo.
The above concerns the methods which so far have been used industrially for the production of epitaxy quality layers.
From German Patent Document DE-OS 36 14 384, a PECVD method is known in which DC glow discharge in the form of a low-voltage discharge is used. As the result, layers which have particularly good mechanical characteristics are to be deposited rapidly, that is at a high growth rate.
A cathode chamber with a hot cathode communicates with a vacuum recipient by way of a diaphragm. An anode is provided opposite the diaphragm. In parallel to the discharge axis formed between the diaphragm and the cathode, an inlet arrangement is provided for a reactive gas. Workpieces are arranged opposite this arrangement with respect to the discharge axis. With respect to the anode potential, discharge voltages UAK below 150 V are applied, and the discharge is operated with a current intensity IAK of at least 30 A. For the coating, the workpieces are brought to negative potentials between 48 and 610 V.
The tests illustrated therein result in the following picture:
The present invention is now based on the recognition that workpiece coatings can be carried out which have a layer quality which meets the demands made on epitaxy layers in that, for this purpose, in contrast to previous expectations, a non-microwave-plasma PECVD method is usedxe2x80x94that is, a PECVD method with DC dischargexe2x80x94and specifically a PECVD method as known, with respect to its principle, from German Patent Document DE-OS 36 14 348. As will be illustrated, it will be possible to achieve in epitaxy quality:
a) Growth rates GR of at least 150 xc3x85/min, even of at least 600 xc3x85/min;
b) GRF of at least 7.5 xc3x85/(sccmxc2x7min), or even 40 xc3x85/(sccmxc2x7min), preferably even 75 xc3x85/(sccmxc2x7min), and further
c) gas utilization numbers GAF at least in the range of 5%.
It is recognized that, in the case of the DC-PECVD method used according to the invention, the plasma discharge leads to the lowest-energy ions, also to the lowest-energy electrons, but that the charge carrier density, particularly the electron density at the utilized discharge is very high.